Method for making lithium-sulfur battery separator

ABSTRACT

The present disclosure relates to a method for making a lithium-sulfur battery separator. The method comprises providing a separator substrate, and forming a functional layer on at least one surface of the separator substrate. A method for forming the functional layer comprises applying a first carbon nanotube layer on the at least one surface; dispersing a plurality of graphene oxide sheets and a plurality of manganese dioxide nanoparticles in a solvent to form a mixture; and depositing the mixture on a surface of the first carbon nanotube layer to form a first graphene oxide composite layer; applying a second carbon nanotube layer on a surface of the first graphene oxide composite layer; and forming a second graphene oxide composite layer on a surface of the second carbon nanotube layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. § 119 fromChina Patent Application No. 201610799889.3, filed on Aug. 31, 2016, inthe China Intellectual Property Office, the contents of which are herebyincorporated by reference. The application is also related to copendingapplications entitled, “LITHIUM-SULFUR BATTERY SEPARATOR ANDLITHIUM-SULFUR BATTERIES USING THE SAME”, filed Jul. 19, 2017 Ser. No.15/653,541.

FIELD

The present disclosure relates to a method for making lithium-sulfurbattery separator.

BACKGROUND

A lithium-sulfur battery cathode is sulfur, and a lithium-sulfur batteryanode is elemental lithium. During electrical discharge process, theelemental lithium loses electrons to become lithium-ion, the sulfurreacts with the lithium-ion and the electrons to produce sulfides. Areaction equation is expressed as follows: S₈+16Li⁺+16e⁻¹=8Li₂S. Alithium-sulfur battery has advantages of low-cost, environmentalfriendliness, good safety, and high theoretical specific capacity.

Separator is an important component of the lithium-sulfur battery. Theseparator is to separate the cathode and the anode to avoid an internalshort-circuit. However, polysulfides formed during an electricaldischarge process have a high polarity and can easily dissolve into anelectrolyte, lithium-sulfur battery separators obtained by conventionalmethods are difficult to inhibit polysulfide diffusion. With a greatloss of active sulfur, a “shuttle effect” would occur betweenelectrodes. Thus the specific capacity and cycling stability of thelithium-sulfur battery would be limited.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations of the present technology will now be described, by wayof example only, with reference to the attached figures, wherein:

FIG. 1 is a structure schematic view of one embodiment of alithium-sulfur battery separator.

FIG. 2 is another structure schematic view of one embodiment of thelithium-sulfur battery separator.

FIG. 3 is a cross-section morphology of one embodiment of a functionallayer of the lithium-sulfur battery separator.

FIG. 4 is a structure schematic view of one embodiment of the functionallayer of the lithium-sulfur battery separator.

FIG. 5 is a surface morphology of the functional layer of thelithium-sulfur battery separator.

FIG. 6 is a scanning electron microscope image of one embodiment of acarbon nanotube layer of the lithium-sulfur battery separator.

FIG. 7 is constant current charge-discharge test curves of alithium-sulfur battery of Example 1 and a lithium-sulfur battery ofComparative Example 1.

FIG. 8 is charge-discharge voltage profiles at different cycles of thelithium-sulfur battery of Example 1.

FIG. 9 is a cyclic stability performance at different charge/dischargerate of the lithium-sulfur battery of Example 1.

FIG. 10 is a cyclic stability performance at charge/discharge rate of0.5 C of the lithium-sulfur battery of Example 1 after a rate test.

FIG. 11 is a prolonged cyclic stability performance of thelithium-sulfur battery of Example 1 and the lithium-sulfur battery ofComparative Example 1 at 1 C.

FIG. 12 is a self-discharge test of the lithium-sulfur battery ofExample 1 after standing for 20 days.

FIG. 13 is a self-discharge test of the lithium-sulfur battery ofComparative Example 1 after standing for 20 days.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “another,” “an,” or “one” embodiment in this disclosure are notnecessarily to the same embodiment, and such references mean “at leastone.”

It will be appreciated that for simplicity and clarity of illustration,where appropriate, reference numerals have been repeated among thedifferent figures to indicate corresponding or analogous elements. Inaddition, numerous specific details are set forth in order to provide athorough understanding of the embodiments described herein. However, itwill be understood by those of ordinary skill in the art that theembodiments described herein can be practiced without these specificdetails. In other instances, methods, procedures, and components havenot been described in detail so as not to obscure the related relevantfeature being described. Also, the description is not to be consideredas limiting the scope of the embodiments described herein. The drawingsare not necessarily to scale, and the proportions of certain parts havebeen exaggerated to illustrate details and features of the presentdisclosure better.

Several definitions that apply throughout this disclosure will now bepresented.

The term “substantially” is defined to be essentially conforming to theparticular dimension, shape, or other feature which is described, suchthat the component need not be exactly or strictly conforming to such afeature. The term “comprise,” when utilized, means “include, but notnecessarily limited to”; it specifically indicates open-ended inclusionor membership in the so-described combination, group, series, and thelike.

Referring to FIG. 1, one embodiment is described in relation to alithium-sulfur battery separator 10. The lithium-sulfur batteryseparator 10 comprises a separator substrate 110 and a functional layer120. The separator substrate 110 comprises a first surface and a secondsurface opposite to the first surface. The functional layer 120 iscovered on at least one surface of the first surface and the secondsurface. In one embodiment, the functional layer 120 is only covered onthe first surface. Referring to FIG. 2, in one embodiment, thefunctional layer 120 is covered on both the first surface and the secondsurface. The separator substrate 110 is located between two functionallayers 120.

The separator substrate 110 can be a microporous polyolefin membrane.The microporous polyolefin membrane comprises a polypropylene (PP) film,a polyethylene (PE) film, or a multilayer composite film of the PP filmand the PE film. The separator substrate 110 comprises a plurality ofmicro pores. In one embodiment, the separator substrate 110 is a PE filmhaving a thickness of 20 micrometers, and a size of each of theplurality of micro pores in the PE film is about 1 micrometer.

A thickness of the functional layer 120 can be ranged from about 1micrometer to about 3 micrometers. Referring to FIG. 3, in oneembodiment, the thickness of the functional layer 120 is about 2micrometers.

Referring to FIG. 4, the functional layer 120 comprises at least twographene oxide composite layers 122 and at least two carbon nanotubelayers 124. The at least two graphene oxide composite layers 122 and theat least two carbon nanotube layers 124 are alternatively overlappedwith each other to form a multi-layer structure, which is located on atleast one surface of the first surface and the second surface. One ofthe at least two carbon nanotube layers 124 is directly contact with theseparator substrate 110. Each of the at least two graphene oxidecomposite layers 122 comprises a plurality of graphene oxide sheets 1222and a plurality of manganese dioxide (MnO₂) nanoparticles 1224. Theplurality of graphene oxide sheets 1222 are flatly covered on the carbonnanotube layers 124. The plurality of graphene oxide sheets 1222 areoverlapped with each other. The plurality of MnO₂ nanoparticles 1224 areuniformly adsorbed on the plurality of graphene oxide sheets 1222, andembedded in an interlayer formed by the carbon nanotube layer 124 andthe graphene oxide sheets 1222. In one embodiment, an amount of thegraphene oxide composite layers 122 is equal to an amount of the carbonnanotube layer 124. In one embodiment, both the amount of the grapheneoxide composite layers 122 and the amount of the carbon nanotube layer124 ranges from about 8 layers to about 12 layers. When the number ofthe graphene oxide composite layers 122 and the number of the carbonnanotube layer 124 are too large, the thickness of the functional layer120 is too large and an energy density of the lithium-sulfur batterywould be reduced; on the contrary, an electrochemical reactivity of thelithium-sulfur battery separator 10 would be poor and a shuttle effectof the polysulfide would occur. In one embodiment, the functional layer120 comprises ten graphene oxide composite layers 122 and ten carbonnanotube layers 124 stacked and alternated with each other.

Referring to FIG. 5, the MnO₂ nanoparticles 1224 are uniformly dispersedon the surface of the functional layer 120, and the graphene oxidesheets 1222 and the MnO₂ nanoparticles 1224 are uniformly distributed inthe functional layer 120.

A diameter of each of the plurality of MnO₂ nanoparticles 1224 can beranged from about 5 nanometers to about 10 nanometers. A weight ratiobetween the plurality of MnO₂ nanoparticles 1224 and the plurality ofgraphene oxide sheets 1222 can be ranged from about 1:2 to about 1:1. Inone embodiment, the weight ratio between the plurality of MnO₂nanoparticles 1224 and the plurality of graphene oxide sheets 1222 isabout 1:1

The plurality of MnO₂ nanoparticles 1224 can be adsorbed on theplurality of graphene oxide sheets 1222 by strong van der Waals force.The size of each of the plurality of MnO₂ nanoparticles 1224 is small. Asize of each of the graphene oxide sheets 1222 is large, and amechanical strength of the graphene oxide sheets 1222 is large.Therefore, the plurality of MnO₂ nanoparticles 1224 can be firmlyadsorbed on the plurality of graphene oxide sheets 1222. In addition,the carbon nanotube layer 124 is a pure structure consisting of carbonnanotubes, and a size of the pores in the carbon nanotube layer 124 isonly a few tens of nanometers; thus, the graphene oxide composite layers122 can be fixed by the carbon nanotube layer 124, and the plurality ofMnO₂ nanoparticles 1224 will not move freely in the pores of the carbonnanotube layer 124.

The carbon nanotube layer 124 can be one carbon nanotube film or atleast two carbon nanotube films stacked with each other. Referring toFIG. 6, in one embodiment, the carbon nanotube layer 124 is one carbonnanotube film, adjacent carbon nanotube layers are crossed with eachother, and an angle between adjacent carbon nanotube layers is about 90degrees. When the carbon nanotube layer 124 comprises at least twocarbon nanotube films stacked with each other, the at least two carbonnanotube films are tightly joined together by van der Waals forces. Inone embodiment, the at least two carbon nanotube films are crossed witheach other, and an angle between adjacent carbon nanotube films is about90 degrees. In one embodiment, the carbon nanotube layer 124 is formedby two carbon nanotube films stacked and crossed with each other, anangle between the two carbon nanotube films is about 90 degrees.

The carbon nanotube film consists of a plurality of carbon nanotubes. Anend of one carbon nanotube is joined to an end of an adjacent carbonnanotube arranged substantially along the same direction by van derWaals force. In one embodiment, the carbon nanotube film is a drawncarbon nanotube film. A large number of carbon nanotubes in the drawncarbon nanotube film can be oriented along a preferred direction,meaning that a large number of the carbon nanotubes in the drawn carbonnanotube film are arranged substantially along the same direction. Aminority of carbon nanotubes in the drawn carbon nanotube film may berandomly aligned. However, the number of randomly aligned carbonnanotubes is very small and does not affect the overall orientedalignment of the majority of carbon nanotubes in the drawn carbonnanotube film.

Oxygen-containing functional groups of the graphene oxide sheets 1222and the MnO₂ nanoparticles 1224 have a strong chemical adsorption onpolysulfides, and the carbon nanotube layer 124 has excellent mechanicalproperties and electrical conductivity. Therefore, the lithium-sulfurbattery separator 10 exhibits superior performance to confine thepolysulfides diffusion, the “shuttle effect” between the cathode and theanode can be avoided; and an electrochemical reactivity of thelithium-sulfur battery with the lithium-sulfur battery separator 10 isenhanced.

One embodiment is described in relation to a method for making thelithium-sulfur battery separator 10. The method comprises the followingsteps:

step (S1), providing the separator substrate 110; and

step (S2), forming the functional layer 120 on a surface of theseparator substrate 110, which comprises sub-steps of:

step (S21), applying a first carbon nanotube layer on the surface of theseparator substrate 110;

step (S22), providing the plurality of graphene oxide sheets 1222 andthe plurality of MnO₂ nanoparticles 1224, dispersing the plurality ofgraphene oxide sheets 1222 and the plurality of MnO₂ nanoparticles 1224in a solvent to form a mixture, and depositing the mixture on a surfaceof the first carbon nanotube layer to form a first graphene oxidecomposite layer;

step (S23), applying a second carbon nanotube layer on a surface of thefirst graphene oxide composite layer; and

step (S24), forming a second graphene oxide composite layer on a surfaceof the second carbon nanotube layer.

In step (S2), the solvent can be a volatile nonpolar solvent, such asethanol and isopropanol. In one embodiment, the solvent is ethanol.

Both the first carbon nanotube layer and the second carbon nanotubelayer are the same as the carbon nanotube layer 124. In one embodiment,both the first carbon nanotube layer and the second carbon nanotubelayer are formed by two carbon nanotube films stacked and crossed witheach other, and an angle between the two carbon nanotube films is about90 degrees.

The carbon nanotube film can be drawn from a carbon nanotube array via astretch tool. The carbon nanotube film is directly laid on the separatorsubstrate 110 after drawn from the carbon nanotube array. In oneembodiment, a height of the carbon nanotube array is about 300micrometers. A diameter of the carbon nanotubes of the carbon nanotubearray can range from about 20 nanometers to about 30 nanometers. Amethod of the drawn carbon nanotube film is taught by U.S. Pat. No.8,048,256 to Feng et al.

When the first carbon nanotube layer comprises more than two carbonnanotube films, a method of applying the first carbon nanotube layer onthe surface of the separator substrate 110 comprises the followingsteps: step (S211), laying a first carbon nanotube film on the surfaceof the separator substrate 110; step (S212), laying a second carbonnanotube film on a surface of the first carbon nanotube film, wherein afirst extending direction of the carbon nanotubes in the first carbonnanotube film intersects with a second extending direction of the carbonnanotubes in the second carbon nanotube film; step (S213), laying athird carbon nanotube film on a surface of the second carbon nanotubefilm, wherein a third extending direction of the carbon nanotubes in thethird carbon nanotube film intersects with the second extendingdirection of the carbon nanotubes in the second carbon nanotube film;and step (S214) repeating above steps until the first carbon nanotubelayer is obtained. When the first carbon nanotube layer comprises twocarbon nanotube films, the method of applying the first carbon nanotubelayer on the surface of the separator substrate 110 only comprises step(S211) and step (S212). In one embodiment, the first carbon nanotubefilm is laid on the surface of the separator substrate 110; then thesecond carbon nanotube film is laid on the surface of the first carbonnanotube film, and the first extending direction is substantiallyperpendicular with the second extending direction.

A method for applying the second carbon nanotube layer on the surface ofthe first graphene oxide composite layer is the same as the method forapplying the first carbon nanotube layer on the surface of the separatorsubstrate 110.

In step (S21), further comprising fixing the separator substrate 110 toa planar glass before applying the first carbon nanotube layer on thesurface of the separator substrate 110.

In step (S22), the plurality of graphene oxide sheets 1222 and theplurality of MnO₂ nanoparticles 1224 are uniformly dispersed in thesolvent by mechanical stirring or ultrasonication. A weight ratiobetween the plurality of MnO₂ nanoparticles 1224 and the plurality ofgraphene oxide sheets 1222 can be ranged from about 1:2 to about 1:1. Inone embodiment, 5 mg graphene oxide sheets and 5 mg MnO₂ nanoparticlesare dispersed in 40 mL solution by intensive ultrasonication for 30 min.

A method for depositing the mixture on the surface of the first carbonnanotube layer comprises the following steps: the mixture is uniformlydeposited on the surface of the first carbon nanotube layer by a dropperor a slow dumping; the first carbon nanotube layer is impregnated by themixture; and the solvent in the mixture is removed by heating.

The plurality of MnO₂ nanoparticles 1224 can be adsorbed on theplurality of graphene oxide sheets 1222 by van der Waals force bymechanical stirring or ultrasonic shock, in addition, the size of eachof the plurality of MnO₂ nanoparticles 1224 is small and the size ofeach of the graphene oxide sheets 1222 is large, and the mechanicalstrength of the graphene oxide sheets 1222 is large; thus, the pluralityof MnO₂ nanoparticles 1224 can be firmly adsorbed on the graphene oxidesheets.

In one embodiment, step (S23) and step (S24) are repeated at least twotimes to form the functional layer 120 comprising at least two grapheneoxide composite layers 122 and at least two carbon nanotube layers 124stacked and alternated with each other. In other embodiments, step (S23)and step (S24) are repeated seven times to eleven times.

In one embodiment, step (S23) and step (S24) are repeated nine times toform a functional layer comprising ten graphene oxide composite layers122 and ten carbon nanotube layers 124 alternatively stacked with eachother. Firstly, laying the second carbon nanotube layer on the surfaceof the first graphene oxide composite layer, and forming the secondgraphene oxide composite layer on the surface; laying the third carbonnanotube layer on a surface of the second graphene oxide compositelayer, and forming a third graphene oxide composite layer on a surfaceof the third carbon nanotube layer; repeating above steps until tencarbon nanotube layers are formed on the separator substrate 110. And,each of the ten carbon nanotube layers is covered by one graphene oxidecomposite layer.

A surface of the separator substrate 110 is defined as the firstsurface, and a surface of the separator substrate 110 opposite to thefirst surface is defined as the second surface. In one embodiment, instep (S2), the functional layer 120 is formed on both the first surfaceand the second surface.

One embodiment is described in relation to a lithium-sulfur battery. Thelithium-sulfur battery comprises a positive electrode, a negativeelectrode, a lithium-sulfur battery separator, and an electrolyticsolution. The lithium-sulfur battery separator is located between thepositive electrode and the negative electrode. The positive electrode isa composite electrode comprising sulfur and carbon nanotubes, and aweight ratio between the sulfur and the positive electrode ranges fromabout 60 wt % to about 80 wt %. The carbon nanotubes have excellentmechanical properties, conductivity, and large aspect ratio, thus thepositive electrode has excellent mechanical properties and conductivitywithout polymer binder or current collector, further improving an energydensity of the lithium-sulfur battery. The negative electrode is ametallic lithium foil. The lithium-sulfur battery separator is thelithium-sulfur battery separator 10.

EXAMPLE 1

In the lithium-sulfur battery of this example, the positive electrode isthe composite electrode comprising sulfur and carbon nanotubes, and theweight ratio between the sulfur and the positive electrode is about 75wt %. The negative electrode is the metallic lithium foil. Theelectrolytic solution is 1 M lithium bis(trifluoromethanesulfonyl)imide(LiTFSI) solution in 1,3-dioxolane (DOL)/Dimethoxyethane (DME) (volumeratio 1:1) with 0.2 M LiNO₃ as additive.

The lithium-sulfur battery separator comprises the separator substrateand the functional layer. The separator substrate is a polyethylene filmhaving a thickness of 20 micrometers. The functional layer comprises tengraphene oxide composite layers and ten carbon nanotube layersalternatively stacked with each other. The plurality of MnO₂nanoparticles are uniformly adsorbed on the graphene oxide sheets, andembedded in an interlayer formed by the carbon nanotube layer and thegraphene oxide sheets. The carbon nanotube layer is formed by ten carbonnanotube films stacked and crossed with each other.

Comparative Example 1

In this comparative example, the lithium-sulfur battery is the same asthat in Example 1, except that the lithium-sulfur battery separator is apolyethylene film having a thickness of 20 micrometers.

Referring to FIG. 7, the lithium-sulfur battery in Example 1 is chargedand discharged at a constant rate of 0.5 C, after a charge/dischargecycle is performed 200 times, a capacity of the lithium-sulfur batteryof Example 1 is about 654 mA h g⁻¹ (relative to the electrode); however,a capacity of the lithium-sulfur battery of Comparative Example 1 isonly 316 mA h g⁻¹ (relative to the electrode). It shows that thecapacity and a capacity retention ratio of the lithium-sulfur battery ofExample 1 are greatly improved compared with the lithium-sulfur batteryof Comparative Example 1.

Referring to FIG. 8, it can be seen that after 200 charge/dischargecycles at a constant rate of 0.5 C, the capacity retention ratio of thelithium-sulfur battery of Example 1 is about 80.4%. After 1st, 50th,100th, and 200th charge/discharge cycles, the lithium-sulfur battery hastwo typical discharge plateaus of 2.35V and 2.10V. It shows that theloss of active sulfur in the lithium-sulfur battery is greatlysuppressed by the lithium-sulfur battery separator of Example 1, the“shuttle effect” is avoided between the cathode and the anode, and theelectrochemical reactivity of the lithium-sulfur battery is improved.

Referring to FIG. 9, a rate test is divided into a first part and asecond part, in the first part, the lithium-sulfur battery of Example 1is discharged at a constant rate of 0.5 C, then charged at 0.2 C, 0.5 C,1 C, 5 C, 7 C and 10 C, respectively. It can be seen that the dischargecapacity of the lithium-sulfur battery remains large and a capacitydecay is quite small under high rate charge current. In the second part,the lithium-sulfur battery of Example 1 is charged/discharged at highrates, it can be seen that the capacity retention ratio of thelithium-sulfur battery is large under high rate charge/dischargecurrent, and a capacity retention is excellent at post low-rate cycles.Therefore, the lithium-sulfur battery of Example 1 has excellentelectrochemical properties.

Referring to FIG. 10, it can be seen that after high ratecharge/discharge test, a long cycle stability of the lithium-sulfurbattery of Example 1 does not fade obviously. After one hundredcharge/discharge cycles at constant rate of 0.5 C, the capacity of thelithium-sulfur battery of Example 1 is about 700 mA h g⁻¹.

Referring to FIG. 11, it can be seen that after 2500 charge/dischargecycles at constant rate of 1 C, a discharge capacity of thelithium-sulfur battery of Example 1 is about 293 mA h g⁻¹, and acoulombic efficiency is up to 98.8%. However, the lithium-sulfur batteryof Comparative Example 1 exhibited an obvious capacity fading process,and an internal short circuit occurs after 700 charge/discharge cycles.In addition, a coulombic efficiency of the lithium-sulfur battery ofComparative Example 1 is lower and has an obvious fluctuation comparedto the lithium-sulfur battery of Example 1.

Referring to FIG. 12, the lithium-sulfur battery of Example 1 still hasexcellent stability after standing for 20 days. A discharge capacityretention ratio is about 93.0%, and the lithium-sulfur battery ofExample 1 shows excellent stability during 100 charge/discharge cyclesprocess. However, referring to FIG. 13, the discharge capacity of thelithium-sulfur battery of Comparative Example 1 has an obvious decayafter standing for 20 days. It can also be seen that during the 100charge/discharge cycles process, the lithium-sulfur battery ofComparative Example 1 has an obvious activation process, but itsdischarge capacity is still not high. FIG. 12 and FIG. 13 show that thelithium-sulfur battery in Example 1 was substantially free fromself-discharge, however, the lithium-sulfur battery in ComparativeExample 1 has serious self-discharge phenomenon.

It is to be understood that the above-described embodiments are intendedto illustrate rather than limit the present disclosure. Variations maybe made to the embodiments without departing from the spirit of thepresent disclosure as claimed. Elements associated with any of the aboveembodiments are envisioned to be associated with any other embodiments.The above-described embodiments illustrate the scope of the presentdisclosure but do not restrict the scope of the present disclosure.

Depending on the embodiment, certain of the steps of a method describedmay be removed, others may be added, and the sequence of steps may bealtered. The description and the claims drawn to a method may includesome indication in reference to certain steps. However, the indicationused is only to be viewed for identification purposes and not as asuggestion as to an order for the steps.

What is claimed is:
 1. A method for making a lithium-sulfur batteryseparator comprising step (S1), providing a separator substratecomprising a first surface and a second surface opposite to the firstsurface; and step (S2), forming a functional layer on at least onesurface of the first surface and the second surface, and a method forforming the functional layer on at least one surface of the firstsurface and the second surface comprises sub-steps of: step (S21),applying a first carbon nanotube layer on the at least one surface ofthe first surface and the second surface; step (S22), providing aplurality of graphene oxide sheets and a plurality of manganese dioxidenanoparticles; dispersing the plurality of graphene oxide sheets and theplurality of manganese dioxide nanoparticles in a solvent to form amixture; and depositing the mixture on a surface of the first carbonnanotube layer to form a first graphene oxide composite layer; step(S23), applying a second carbon nanotube layer on a surface of the firstgraphene oxide composite layer; and step (S24), forming a secondgraphene oxide composite layer on a surface of the second carbonnanotube layer.
 2. The method of claim 1, wherein the first carbonnanotube layer comprises at least two carbon nanotube films stacked andcrossed with each other.
 3. The method of claim 2, wherein the firstcarbon nanotube layer comprises two carbon nanotube films stacked andcrossed with each other, and a method of applying the first carbonnanotube layer on the at least one surface of the first surface and thesecond surface comprises the following steps: laying a first carbonnanotube film on the at least one surface of the first surface and thesecond surface; and laying a second carbon nanotube film on a surface ofthe first carbon nanotube film, wherein a first extending direction ofthe carbon nanotubes in the first carbon nanotube film intersects with asecond extending direction of the carbon nanotubes in the second carbonnanotube film.
 4. The method of claim 2, wherein the first carbonnanotube layer comprises more than two carbon nanotube films stacked andcrossed with each other, and a method of applying the first carbonnanotube layer on the at least one surface of the first surface and thesecond surface comprises the following steps: step (S211), laying afirst carbon nanotube film on the at least one surface of the firstsurface and the second surface; step (S212), laying a second carbonnanotube film on a surface of the first carbon nanotube film, wherein afirst extending direction of the carbon nanotubes in the first carbonnanotube film intersects with a second extending direction of the carbonnanotubes in the second carbon nanotube film; step (S213), laying athird carbon nanotube film on a surface of the second carbon nanotubefilm, wherein a third extending direction of the carbon nanotubes in thethird carbon nanotube film intersects with the second extendingdirection of the carbon nanotubes in the second carbon nanotube film;and step (S214), repeating step (S212) and step (S213) until the firstcarbon nanotube layer is obtained.
 5. The method of claim 1, wherein thefirst carbon nanotube layer is directly laid on the separator substrateafter drawn from a carbon nanotube array.
 6. The method of claim 1,wherein in step (S21), further comprising fixing the separator substrateto a planar glass before applying the first carbon nanotube layer on theat least one surface of the first surface and the second surface.
 7. Themethod of claim 1, wherein in step (S22), the plurality of grapheneoxide sheets and the plurality of manganese dioxide nanoparticles areuniformly dispersed in the solvent by a mechanical stirring.
 8. Themethod of claim 1, wherein in step (S22), the plurality of grapheneoxide sheets and the plurality of manganese dioxide nanoparticles areuniformly dispersed in the solvent by an ultrasonic shock.
 9. The methodof claim 1, wherein a weight ratio between the plurality of manganesedioxide nanoparticles and the plurality of graphene oxide sheets isranged from about 1:2 to about 1:1.
 10. The method of claim 1, wherein amethod of depositing the mixture on the surface of the first carbonnanotube layer comprises the following steps: the mixture is uniformlydeposited on the surface of the first carbon nanotube layer; the firstcarbon nanotube layer is impregnated by the mixture; and the solvent inthe mixture is removed by heating.
 11. The method of claim 10, whereinthe mixture is uniformly deposited on the surface of the first carbonnanotube layer by a dropper.
 12. The method of claim 10, wherein themixture is uniformly deposited on the surface of the first carbonnanotube layer by a slow dumping.
 13. The method of claim 1, whereinstep (S23) and step (S24) are repeated seven times to eleven times. 14.The method of claim 1, wherein a diameter of each of the plurality ofmanganese dioxide nanoparticles is ranged from about 5 nanometers toabout 10 nanometers.
 15. The method of claim 1, wherein a thickness ofthe functional layer is ranged from about 1 micrometer to about 3micrometers.